Cardiac and cardiovascular imaging using magnetic resonance imaging (MRI) techniques is of intense interest. The use of MRI has obvious advantages over other imaging techniques which typically employ radiation, such as X-rays. However, for cardiac studies, the subject is often required to remain within the MRI magnet for at least thirty to sixty minutes. During this period, it is advisable to observe the electrocardiogram (ECG) of the patient continuously, especially if imaging is being used to diagnose or study potential cardiac conditions. Moreover, many imaging protocols include stressing the cardiac system by, for example, pharmacological methods, to view the heart or other portions of the system under stress. Again, it is advisable to monitor the heart under these conditions to improve the diagnosis and to improve the safety of the imaging techniques (e.g., to indicate if the stressing of the cardiac system is causing potentially dangerous or damaging results.)
The electrocardiogram typically is presented in the form of a potential difference between two electrodes measured as a function of time. The potential difference arises from the electrical signals generated by the heart during its cardiac cycle. The heart's electrical discharge spreads through the conducting tissues of the body to the body surface where it can be monitored using the electrodes. A typical cardiac cycle is presented in FIG. 1. The QRS portion of the cardiac electrical signal represents ventricular depolarization of heart cells, which causes the ventricles to contract. The ST portion of the ECG corresponds to the repolarization or resetting of the electrical system after the contraction. By observation of the form and amplitudes of these and other portions of the ECG, various cardiac conditions can be diagnosed.
The monitoring of the ECG during MRI procedures is complicated by the presence of the magnet and its associated high intensity magnetic field, as well as the gradient fields and rf sampling fields used in MRI techniques. Furthermore, during an MRI procedure, the blood of the patient become magnetically polarized due to the magnet and rf fields. The blood acts as a moving conductor and generates magnetohydrodynamic potentials as the blood flows through the body. Moreover, these potentials are constantly changing as the velocity of the blood flow changes over the course of the cardiac cycle (i.e., the velocity of the blood increases as it is pumped and then slows over time until it is pumped again).
The potentials generated by the blood flow disrupt, distort, and/or overwhelm the cardiac signals as received by the ECG electrodes. This greatly reduces the diagnostic quality of the ECG signals. While attempts have been made to reduce the direct interference of the rf and gradient fields by, for example, shielding, filtering and/or monitoring of the ECG only during quiescent periods, there has not been a satisfactory method or device for addressing the effect on the ECG of the magnetohydrodynamic potentials arising from the flow of blood.